1. Field of the Invention
This invention relates to the flatness examining device designed to display the interference fringe of the reflecting lights that are reflected at the reference plane of the optical member and at the measured plane of the sample, with the aid of the oblique-incidence interference method.
2. Description of the Prior Art
As known well, the oblique-incidence interference method is designed that with the use of two media being separated by the reference plane and having different refractive indices, a paralleled light flux is obliquely incidented, through said media, onto the measured plane of the sample that is placed in parallel to the reference plane. The optical-path difference of the reflecting lights, which reflected at the reference plane and at the measured plane, would produce the interference fringe. The state of the fringe will aid to know the flatness on the measured plane.
As an example for the flatness examining device, to which the oblique incidence interference method is applied, there is a type as described in Japanese Patent Examined Publication No. 074/1978, that the interference fringe is displayed through an image formation lens on a screen. This interference fringe represent the pattern of concave or convex state of the measured plane. The state of the measured plane, concave or convex, can be determined based on the observation of this pattern.
In the past, the state of the measured plane, concave or convex, was determined by a light push given to the sample in the manner that if the interference fringe expand inward in increased quantity, the measured plane would represent concave, and if the fringe expand outward in reduced quantity, the measured plane represents convex.
However, direct push onto the sample would cause a flaw or dirt on the surface of the sample, or relatively large pus would largely change the degree of variation of concave or convex state. Further, if the sample is large, difference between the variation caused at a push point and the variation at a distant point would result in difficulty or retardation for determination.